1. Field of the Invention:
This invention relates to a recording medium for optically recording signals of various information such as video information and apparatus for optically recording and reproducing information thereon and therefrom. And more particularly to a recording medium on which information can be recorded at high density, and recording/reproducing apparatuses therefor.
2. Description of the Prior Art:
Generally, a recording medium is formed as a recording disc which includes a center hole at its central portion, and information are recorded as a single track in a spiral form surrounding the outer periphery of the center hole. Such a recording disc includes, as shown in FIG. 1A, information pit portion 10 formed on a transparent disc substrate or base 15 such as Polymethyl methacrylate (PMMA), an aluminum film 12 of a high reflection factor laminated on the disc substrate 15 which is provided with the information pit portion 10 thereon, and a protective layer 11 such as plastic formed on the aluminum film 12.
In the case of reproducing information from the recording disc, a light beam LB such as laser beam is irradiated from the transparent disc substrate 15 side onto information pit portion 10 of a track section to be reproduced. As a result, as shown in FIG. 15B, rays of reflected light LB.sub.R reflected at mirror-finished surface portions between pits where no information pit portion 10 exists are detected as a "bright" light, and rays of reflected light LB.sub.R reflected at the portion of information pit portion 10 are detected as a "dark" light. In this way, information corresponding to respective recording pits 20a, 20b, 21, . . . of the information pit portions 10 will be reproduced by rays of reflected light of "bright" or "dark" light.
According to the recording disc and the reproducing method as described above, rays of reflected light are detected as a signal light optically amplitude-modulated by a change in the reflection factor of a light beam of a read-out light emitted onto the information pit portions. As a result, binary signals are recorded and reproduced in correspondence with the obtained signal light of "bright" or "dark". Since information is merely recorded as a binary signal of one bit in a manner of the presence or absence of information pits as described above, there is a limit in the information integration density per unit area of the disc. For this reason, there was the problem that it is difficult to increase the information integration density to be high. Further, even if information is recorded at high density by using any other recording method such as reducing the track pitch, etc., it was difficult to precisely reproduce the recorded information because the light beam is simultaneously irradiated on the adjacent information pits as shown in FIG. 1C.
As a technology to solve the above-mentioned problem, there is a recording carrier (recording medium) disclosed in the Japanese Patent Application Laid Open No. 119517/1975. In this recording medium, at least two kinds of diffraction gratings are provided in areas arranged in a track form so that directions of the grating lines are different for every kinds of diffraction gratings. That is, if the two kinds of diffraction gratings are alternately formed on every adjacent tracks, adjacent tracks positioned on both the sizes of the target track are formed by the diffraction grating of other kind than that of the target track and serves as an intermediate zone at the time of reading the target track. Therefore, information recording density becomes substantially twice greater than that of an ordinary recording medium.
However, in the case of the above-mentioned recording carrier, since the content of information is recorded by means of plural kinds of diffraction gratings, the diffraction directions are different for every kinds of the diffraction gratings at the time of reproduction. For this reason, there is the problem that it is necessary to provide a plurality of signal light detection units for each kind of diffraction directions.